1. Field of the Invention
This invention relates to information processing systems. More particularly, the invention relates to power supplied buses for peripheral devices coupled to such systems in which in-rush current is limited to such peripheral devices after power-up or "hot plugging".
2. Background Discussion
In information processing systems where a power supply provides operating current to peripheral devices, two events can occur where a significant amount of current can flow to the peripheral device from the supply. These high currents may damage components in the power supply and also place a great deal of stress on the cable system and connector linking the peripheral device to the power supply.
One event for high current flow occurs in doing power-up of the system. The second event occurs when a peripheral device is plugged-in to the system which is already turned on, commonly referred to as "hot plugging". The high current flow is due, in part, at least to the pre-charging of capacitors in the peripheral device. The prior art has dealt with this problem by introducing current-limiting devices such as current-limiting resistors and switches that provide a lower resistance path in parallel with the current-limiting resistors. Such devices should also maintain the supply voltage for the device circuitry when a surge of current is required by the device which would otherwise cause the supply voltage to decrease or a voltage drop due to distribution losses.
Prior art related to power-up or "hot plugging" of peripheral devices is as follows:
U.S. Pat. No. 5,187,653 discloses a current-limiting circuit able to react to in-rush current surges during start-up. A current monitoring control circuit activates a bypass switch "on" and "off" to assure that excessive current is not allowed to go around a current limiter.
U.S. Pat. No. 5,376,831 issued Dec. 27, 1994, discloses a circuit switch which prevents current surges during initial power applications or during hot-plugging. The circuit includes a switch and transistor in parallel with a resistor combination in series with a capacitor. The circuit controllably supplies initial current from a power supply to a load to prevent a current surge when the switch is hot-plugged or when power is first applied to the switch circuit. A timer changes the voltage across an output transistor to begin a limited conduction of current. The current increases in a controllable manner until the output transistor is turned "on" full to supply operating current to the peripheral device.
All of the prior art places the current-limiting resistors and switches in the main path supplying power to the peripheral device which requires these switches to carry the entirety of the power needed by the peripheral, lowering the efficiency of the peripheral device, during normal operation.